1. Technical Field
The present invention relates to methods for measuring overlay errors of facing objects, and more particularly to a non-contacting method for detecting overlay errors in X and Y axes between two objects facing each other, and detecting a vertical distance between the two objects (in Z axis) as well as parallelism therebetween.
2. Description of Related Art
As to the conventional lithography process, in course of transferring a pattern of a photo mask to a circuit substrate and transferring a pattern predefined on a photo mask to a wafer, measures for error measuring and object aligning are implemented so as to ensure accurate transfer of the patterns. On this note, X-Y positioning, Z positioning, and parallelism between two objects to be mutually overlaid are important. In addition to the lithography process, in course of installing a probe to a superconducting quantum interference microscope (SQUID Microscope), the relative distance and the parallelism between the probe and the assay sample have to be accurately controlled in order to ensure proper sensitivity of the device.
Herein the lithography process is described as an example for illustrating how the prior art deals with the need of three-axis positioning.
Using image capturing devices on the Z axis to monitor overlay between XY-axis marks of overlaid objects is known. Therein, a movable camera may be used to monitor images of overlaid objects in its focal plane and thereby achieve the Z-axis monitoring process. During the process, once the focus changes in the focal plane, it indicates that an inclining angle exists between the two facing objects. However, this known approach is infeasible to get the inclining angle accurately when the image definition of the camera is poor.
Alternatively, two or more image capturing devices are arranged to take images from different view angles for comparison in order to obtain the distance in the Z axis. Nevertheless, this approach needs complicated calculation and operation, yet is defective in accurately giving the inclining angle between overlaid objects.